The present invention relates to optical disk media and optical information recording apparatus which use light to write/read information to/from recording media. In particular, the invention relates to an optical information recording apparatus capable of correcting the sphericalh aberration.
With the progress in density of optical disks and other optical recording media, a typical optical system is employing a larger numerical aperture with shorter wavelength light. Accordingly, aberration due to manufacturing errors of lenses and recording media is becoming a critical problem. For example, the spherical aberration due to the thickness error of a cover layer on a recording medium sharply increases in proportion with the fourth power of the numerical aperture if wavelength is fixed. In addition, the influence of aberration becomes larger in reverse proportion with wavelength λ if manufacturing errors and other conditions are the same.
Owing to these factors, recent high density optical information recording apparatus are required to have optical heads provided with spherical aberration correcting capability. Note that in this specification, “optical information recording apparatus” is used to refer to an apparatus which records or reproduces information to or from a recording medium by using the means to irradiate light to a recording medium and the means to detect light from the recording medium.
In the optical information recording apparatus, it is necessary to automatically adjust both focus and spherical aberration of the lens at the same time. If the spherical aberration is adjusted (and changed), the apparent focal point (optimum focal length) of the lens changes a little. (Hereafter, we refer to the intentional change of the focal point (focus offset) as ‘defocus.’) To reach an optimum condition, it is therefore necessary to change both spherical aberration and defocus concurrently.
If either spherical aberration or defocus is not optimized, the beam spot cannot be reduced enough to allow high-density recording. In addition, manufacturing fluctuations may add astigmatism to the lens and positional errors to the photodetector. In this case, the zero point of the detection signal is deviated, making it impossible to rely on the signal voltage in determining the optimum condition. Therefore, a two-dimensional search or changing both spherical aberration and defocus concurrently is necessary to find the optimum condition. (Hereafter, we refer to the amount of defocus as ‘defocus value.’)
As such a two-dimensional adjustment method of the spherical aberration and defocus, Japanese Patent Laid-open No. 2002-324328 has proposed a method for two-dimensionally searching the respective values in four or eight rectangular directions. However, this method carries the disadvantage that the search speed is slow and a long waiting time is imposed until a medium becomes available after inserted into the optical disk apparatus (drive).